Sewage treatment plant operations usually generate significant quantities of residual sludges. Million of tonnes of sewage sludges are produced worldwide every year. The treatment and final disposition of residual sludges often constitutes the most expensive stage in the treatment of municipal wastewaters and remains a difficult environmental issue.
The main concern in the disposal of the sludge is the presence of toxic heavy metals in the sludge. Heavy metals are concentrated in sewage sludge during the treatment of sludges due to various physic-chemical and biological interactions. The heavy metal content of sewage sludge is about 0.5 to 2% on a dry weight basis. In some cases, extremely high concentrations (up to 4% w/w) of Cd, Cr, Cu, Ni, Pb and Zn have been reported.
Several optional methods for the disposal of municipal sludges could be utilized such as land filling, incineration, oceanic dumping and spreading on agricultural and/or forest lands as fertilizer.
However, the presence of pathogenic microorganisms and elevated concentration of heavy metals present a serious constraint to the implementation of these practices.
Disposal of heavy metal contaminated sludge by these practices pose the following potential risks and problems:
1) Disposal of the metal contaminated sludge in the marine environment involves the danger of accumulation of heavy metals in marine species especially in those which are used for human consumption. PA1 2) High levels of heavy metals released from the sludge can have toxic and lethal effects on marine life. PA1 3) Incineration is costly due to high energy consumption. Moreover heavy metals which remain with the incinerator ashes must be removed before the ash is finally disposed. PA1 4) The biggest risk associated with land filling of the metal contaminated sludge and/or ash, and spreading of the sludge on agricultural and/or forest lands is leaching of heavy metals into surface and ground waters. These may have serious consequences where these waters are used for drinking/recreation by humans and animals. PA1 5) Disposal of the sewage sludge on agricultural land is one of the most economical means of disposal because of its characteristics of being a good fertilizer. However, uptake of heavy metals by plants and the subsequent accumulation of metals in the food chain via plants and animals can be a potential health hazard. PA1 1) Chemical methods which are often associated with consumption of acids (H.sub.2 SO.sub.4, HCl, HNO.sub.3, acetic acid and EDTA) are unattractive due to high cost owing to large acid (0.5 to 0.8 g of H.sub.2 SO.sub.4 per g of dry sludge) and lime requirements. Operational difficulties including the requirement of acid-corrosion resistant apparatus, and safe storage and transportation facilities for acid put constraints on its utilization (Tyagi et al., 1988; Envir. Pollut. 50, 295-316). PA1 2) Removal efficiencies for Fe, Zn, Ni and Cr in 24 hours were more than 76% in the acid treatment methods but Cu could not be solubilized. PA1 3) Combination of heating (95.degree. C.) and 2.0 to 3.5 in 10 to 60 minutes) improved Zn and Ni solubilization but Cu could not be solubilized. PA1 4) Microbial process (in terms of chemical requirements) is 80% cheaper than chemical processes but requires 10 to 14 days (batch time) of incubation time to remove heavy metals from sewage sludge at initial pH 4.0. PA1 5) In the microbial process, addition of ferrous sulphate (up to 20 g/L) enhanced the solubilization of heavy metals to reduce the bioreaction time to 3-4 days (batch time) in the presence of T.ferrooxidans. Moreover, acid addition was also required to adjust pH 4.0. PA1 6) Bioreaction time was reduced to as low as 0.75 day using T. ferrooxidans in a continuously operated bioreactor with 20% sludge recycle. However, to operate the system continuously is very difficult. Separation and cell (sludge) recycle is also a tedious task due to practical problems. PA1 7) Elemental sulphur is a cheaper substrate but most of the S-oxidizing organisms decrease pH of the synthetic medium (9 k with 0.5 g/L K.sub.2 HPO.sub.4) at a maximum rate of 0.4 units per day (15 to 23 days). T.ferrooxidans and T.thiooxidans lowered the pH of sewage sludge from 5.5 to 1.0 in 32 days at 1% sulphur level (Bryant et al., 1983, Can. J. Microbiol. 29, 1159-1170), so very long bioreaction time was required. PA1 8) Most of the acidophilic S-oxidizing bacteria require an initial pH of 4.0 to start the S-oxidation reaction. Therefore, acid is required to lower the pH of the sludge and hence increasing the cost. PA1 9) Sulfolobus acidocaldarius is a fast S-oxidizing organism but require high temperature (55.degree. to 85.degree. C.) for the growth (Brock et al., 1972, Arch. Microbiol., 84, 54-68). PA1 a) lowering, under aerobic conditions, the pH of a municipal sludge from about 7 to 8 to about 2.5 to 2.0 by treating said sludge with 10% by volume of an inoculum containing a mixture of microorganisms comprising Thiobacillus thiooxidans ATCC 55127 and Thiobacillus thioparus ATCC 55128 with 1 to 3 g per liter of sulphur, and incubating for a period of at least 40 hours where the heavy metals originally present in said sludge have been substantially solubilized and the concentration of indicator bacteria have been lowered; and PA1 b) recovering the metals from the low pH treated sludge of step a).
Epidemiological studies to determine disease transmission following the use of sludges as a fertilizer were conducted. These studies have shown that risk of infection is associated with the presence of pathogenic bacteria and helminthic worms.
To prevent environmental pollution and health risks by heavy metals, the content of heavy metals of sewage sludge must be reduced up to a level recommended by the guidelines of various regional municipalities and local governments. The reduction of heavy metals in sewage sludge can be achieved; (I) by source control of discharge to sewer systems or (II) by removing metals from sludge. In source control, the major difficulty resides in the identification of the sources. Moreover, even with complete elimination of toxic metals from all industrial discharges to sewers, the problem remains because of the metal content of domestic wastewater and runoff water. Therefore, to reduce environmental pollution and health hazards by heavy metals from sewage sludge, the heavy metals must be removed before its final disposal. Since large quantities of sludge are generated every day, the process of heavy metals removal should be easy to operate as well as rapid and economical.
Several chemical processes for the removal of heavy metal have been proposed (Hayes et al., 1980, Proc. 34th Ind. Waste Conference, Purdue University, West Lafayette, Ind., 529-543; Jenkins et al. 1981, J. Water Polution Control Fed., 53, 25-32). The acid leaching has resulted in efficient extraction of metals, however, the high operational costs have prevented its widespread use.
Recently, studies have been undertaken to extract metal from wastewater sludges using Thiobacillus ferroxidans, a microorganism which had been successfully used to recover metals from mine tailings. However, the requirement of lowering the initial sludge pH to 4.0 to favour the growth of T. ferrooxidans, use of cell recycle system in a reactor, and the addition of ferrous sulfate as substrate has increased the operational costs of this approach.
There are several short comings in the available chemical and microbial methods to remove heavy metals from sewage sludge. They are summerized as follows:
It would be highly desirable to have a process which could concurrently remove metals and destroy indicator bacteria (fecal streptococci, fecal coliforms, total coliforms) in municipal sludges to such levels which are compatible with agricultural use of the sludges and without all the above-mentioned drawbacks.